1. Field of the Invention
This invention relates to a process for producing highly water-absorptive polymers.
The polymers obtained by the present invention have the highest water absorption capacity ever available not only with respect to pure water but also with respect to various aqueous electrolytic solutions such as physiological salt solution and artificial urine, and are of increased gel strength. Thus, they can advantageously be used for various water-absorptive articles, especially in sanitary fields.
2. Background Art
Highly water-absorptive polymers are synthetic polymers which have recently been used not only for sanitary goods or paper diapers in sanitary fields but also for water retentive materials, dew condensation preventive materials, freshness retentive materials and solvent dehydrating materials in industrial fields as well as in agricultural and horticultural fields, and are now expected to be applied in a wider range of fields.
As such highly water-absorptive polymers, there are known hydrolyzates of starch/acrylonitrile graft copolymers, crosslinked products of carboxymethylcellulose, crosslinked products of polyacrylic acid (or its salts), acrylic acid (or its salts)/vinyl alcohol copolymers, crosslinked products of polyethylene oxide and the like.
In general, the water absorption capacity of a highly water-absorptive polymer can be conceptually expressed by the following equation: ##EQU1##
From this equation, it is apparent that the lower the crosslinking density, the higher the water absorption capacity. In the production of highly water-absorptive polymers using acrylic monomers such as acrylic acid and its alkali metal salts as the starting materials, self-crosslinking tends to proceed excessively even without crosslinking agents whereby the resulting polymers often exhibit insufficient water absorption capacity.
The self-crosslinking may be suppressed to some extent by applying moderate polymerization conditions. However, it is then required to exercise sophisticated control over the polymerization conditions, posing another problems in connection with reproducibility, when taking industrially stable production into account.
Thus, in the production of highly water-absorptive polymers with the use of acrylic monomers as the starting materials, self-crosslinking, which is not yet clarified for its mechanism, forms a barrier against water absorption capacity and reproducibility.